1. Field of the Invention
The present invention generally relates to methods of printing images by using printing devices, and particularly relates to a method of representing halftones of images by forming dots through area-based tone representation.
2. Description of the Prior Art
Printing devices which form letters and images by printing dots represent halftones by adjusting a ratio of a dot-occupied area to a unit area. For example, a square area of 8.times.8 dots or 16.times.16 dots is treated as one unit, and halftones are represented by adjusting the number of dots printed within this area. When deciding whether to print a dot at a given dot position within this area, a level of the input image data is compared with a threshold value allocated to the given dot.
When a binary-value printer is employed, for example, use of an 8-by-8 matrix can represent 64 halftones, and use of a 16-by-16 matrix can represent 256 halftones. In general, area-based halftone representation uses an M-by-N-dot matrix to represent M.times.N.times.(k-1)+1 halftones, wherein k is the number of levels which one dot can represent.
In halftone representation, a decision has to be made with respect to each level of image data as to how many dots are formed in which position of a matrix. A dither matrix is a typical example of a matrix which is used for making such a decision. By defining a dither matrix, a tone curve is created to define a relationship between levels of the input image data and print densities.
In general, it is preferable to set characteristics of a tone curve such that the tone curve can properly represent input images in accordance with characteristics of the input image. For example, a histogram of levels may be obtained with regard to image data, and a tone curve may be defined so as to have a greater dynamic range of halftone representation in levels in which a greater number of pixels are in existence. Such a tone curve can enhance printing characteristics. This technique is disclosed in Japanese Patent Laid-open Applications No.1-237142 and No.63-181568.
The technique of the above-cited references, however, has a problem in that the tone curve defined based on the input image data does not reflect characteristics of printing devices.
Printers generally form dots which have a diameter larger than a dot pitch, and a conventional setting is that the dot diameter is 1.4 times larger than the dot pitch. The use of this dot diameter ensures that adjacent dots are properly connected when a diagonal line is formed with a one-dot line width. If the dot diameter is equal to the dot pitch, a diagonal line appears as a dotted line, and small white gaps are created when all areas are supposed to be printed in black. In order to avoid these, the dot diameter is set so as to be slightly larger than 1.4 times the dot pitch.
Use of a dot diameter larger than the dot pitch creates a problem in halftone representation when printing devices are used.
When a 2-by-2 matrix is used for halftone representation, for example, three halftones can be represented between 0% and 100% of the tone, i.e., halftones of 25%, 50%, and 75% are represented. In order to represent the halftone of 25%, for example, one of the four dots is printed. If the dot diameter is 1.4 times as large as the dot pitch, however, forming one dot in an attempt to achieve an area ratio of a dotted area to the total matrix area being 0.25 will result in an area ratio of 0.385. By the same token, an intended area ratio of 50% will result in an area ratio of 0.77, and an intended area ratio of 75% will end up bringing about an area ratio of 1.02.
Accordingly, a tone curve needs to be defined by taking into consideration a dot diameter of a printing device when halftone representation is employed.
Further, the number of dots printed in halftone representation should be decided based on actually printed lightness. Lightness, which is a measure derived from sensitivity characteristics of human vision, is supposed to be 100 when a surface is a perfectly diffuse reflector, and is zero when there is no reflection. Ideally, white on a printed surface corresponds to lightness of 100, and black on the printed surface is equal to lightness of zero. In actual printing devices, however, white with no printed dot corresponds to lightness of a paper surface, and approximately correspond to lightness of 90. Further, black created by forming all the dots corresponds to lightness of the black ink, and is approximately equal to lightness of 20.
Accordingly, a tone curve needs to be defined by taking into consideration actual lightness on a surface printed by a printing device when halftone representation is employed.
Accordingly, there is a general need to represent halftone images by using a tone curve which takes into account characteristics of the images and characteristics of a printing device that employs area-based halftone representation based on formation of dots.
There is a more specific need to represent halftone images by using a tone curve which takes into account a dot diameter of a printing device that employs area-based halftone representation based on formation of dots.
There is another need to represent halftone images by using a tone curve which takes into account actual lightness generated by a printing device that employs area-based halftone representation based on formation of dots.